The present invention relates to displays and more particularly to an apparatus and technique for protecting predefined display areas in a graphic video display.
In the prior art, video displays are used in a variety of applications to display image information in alphanumeric as well as graphic form. Television and similar cathode ray tube (CRT) type displays are only one example of the many applications which employ display techniques designed to display such information. Such displays rely on the well-known technique of raster scanning a CRT defined by rows and columns of phosphor pixels each of which is modulated by an electron beam to define the picture image. As is known, the raster scanning is accomplished by horizontally scanning each pixel with the electron beam as the beam is indexed vertically from one row to the next until all of the pixels defining the face (screen) of the CRT have been scanned. The scanning is at a repetition rate which has the effect of producing an image on the face of the CRT which changes with time in accordance with the information provided to the electron beam as it scans each pixel. The scanning may be accomplished using well-known interleave techniques which prevent flicker yet produce the same effect as if each line was scanned in sequence to address each of the pixels on the screen.
In employing the above known display techniques, raster scanning has been identified as one of the primary display techniques. Also known however, is the use of stroke writing on the same CRT display. Stroke writing is somewhat different in that it traces only the pixels forming an image by specifically controlling the deflection of the electron beam in accordance with the configuration of the image to be displayed. This is in contrast to the raster display wherein the electron beam scans all pixels horizontally and vertically in a predetermined fashion but modulates the beam at specific pixels during scanning to produce the desired image. In either of such well-known techniques, the configuration of the displayed image is produced by the storage of information which controls the modulation of the electron beam for each screen pixel during raster scan, or which controls the movement and modulation to produce stroke writing. In some cases, display techniques use a combination of raster scanning and stroke writing, wherein the stroke writing is performed during the vertical retrace between scans to produce the benefits of both raster and stroke writing in a display environment.
Although raster scanning and stroke writing techniques have been described above with respect to a CRT, similar techniques may be used in connection with other displays including LCD, TFEL, etc., wherein the scanning and energization of individual visual display elements of a given display produces the desired visual image for the particular type of technology employed. Examples of circuitry for controlling the scanning and display of information are well-known in each of those technologies and need not be described in great detail. However, regardless of the technology employed to provide the visual display, the images which are produced are the result of information storage of sufficient detail to provide actuation of display elements during each scan period so that the desired image may be produced and modified on a scan to scan basis to produce the display which the image represents. In all cases, such displays usually employ a significant amount of memory and control circuitry to provide the original and updated information for each screen scan to be sure that the image is reproduced accurately for visual display.
In many applications, particularly those in which a raster scanned CRT display may be employed, there are certain configurations of images which are repeatedly provided in connection with a display and particular areas that may be designated and protected for only writing such image information. By way of example, in many avionic systems there may be a need to continuously display a certain image representing fixed information which provides a visual display of certain flight data. In still other instances, it may be necessary to provide a fixed background of information which is used in conjunction with certain stroke-written information in other areas of the display such that the stroke-written information may be continually updated while the fixed images are employed. In these cases, the fixed images remain the same for each raster scan of the CRT screen and the only information that changes is that which is being updated by stroke-writing in other areas of the screen. As a result, the screen may be viewed as an image area divided into portions in which no image is written and portions in which writing of information will occur.
In still other instances, it may be desirable to provide an area in which no information is written at all, such that that area of the particular display screen can be protected from writing regardless of the information that may be in the writing control memory. In implementing such systems using conventional technology, large amounts of memory are required to initially store, update and transfer to a writing control, the information necessary to maintain the fixed images and protected areas desired in a given display situation. As a result, there is a continuing need for developing systems and techniques which reduce the memory, complexity, and speed of operation, yet still provide an ability to maintain fixed images and protected areas in a visual display system.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide a video system which allows more simplified control of fixed image generation and display protection.